The more creative the encounter is the more ways you have to adjust on the fly and the less CR is accurate. Sure it gives you an idea on which ballpark you are in.
For example you have multiple lair actions and a selection of those activate each round. If the PCs are hurting then you can choose a few soft ones to give then a chance to regroup and recover. If reinforcements are coming perhaps there are a few less. Similarly if the PCs are cruising, hit 'em hard.
Whether or not this crosses the border into DM fiat/fudging I'll leave up to you.
Also remember you don't have to use the highest DPR actions in general.
Heck, I'd support regular testing for everyone. Laws change, best practices change and regular testing is a good way to keep most up to date. It would cost, be a hassle and extremely unpopular. But if it saves lives and/or make traffic softer it is worth it.
4-5 degrees? You are optimistic. I bet I get to see 3 degrees in my lifetime as we will blast by each and every exit ramps. Not only that we'll also be drifting on the highway, because it looks cool.
All my campaigns are more or less villainous. I encourage my players to go hard on their characters' motivations and goal. I offer easy solutions that hurts people, solutions that put them at odds with society and its norms. One game a character released an ancient enemy of the people in order to advance their own religious doctrine. Another they sold out a band of troll hunters to the trolls for wagon loads of treasure. That event also released the troll king from its prison of chimes sparking a troll march. Final example, the campaign I'm sketching on currently will have the party be a part of a caravan expedition. Peeling back barely a layer and you'll find imperialism and colonialism motivating the existence of the caravan.
So yeah, going villainous is very doable. The thing however is to figure out what will stop the characters from just killing everyone in their way. Easiest is to make sure there are powers more powerful than them and the fear of them keeping the characters discreet. Or they have to follow some rules to keep benefits from some organisation. Or morality? Who knows it may be that the character's gran-gran disapproves of killing and the character doesn't want to upset her.
And as always - talk to your players. You as a group probably can figure out what will keep your characters in line.
Because it isn't western cristianity and/or capitalism. No questioning what power structures comes with it, no questioning of its moralities and ethics. But there are some good things in Islam, just as there are good things in any organized religion. Negatives outweigh however, as the negatives in any organized religion outweigh the positives. Especially in proselytizing religions.
If there is still danger and drama left in the encounter I would agree with you that tossing out HP for "rule of cool" isn't a that good of a practice. That mook with 1hp left can still make an impact.
But if things in essence is over, that the PCs are just cleaning up, then do go ahead and let those remaining hp slide. The outcome is already certain, why drag it out? Wrap it up and move on.
Related is to consider adding morale rules. That one hp mook who moments before had its friend cleaved in one swing may falter and surrender/run away. The result is the same as if the last hp was handwaved away but doesn't feel as much of cheating. Also surrendered foes tend to drive the narrative forward much more than alive ones.
Also because Tolkien wrote it like that.